This invention is directed to current switches, and in particular, to current switches for batteries.
In order to extend the life of rechargeable lead acid storage batteries, it is important that they be switched off if their operating voltage drops below a predetermined level. It is equally important that they also be switched off when the load current is higher than a predetermined design value due to a short circuit or some other adverse condition.
A large number of circuits have been developed for use with batteries to monitor and control their charging and discharging cycles. Among these are U.S. Pat. No. 3,543,043 which issued to D. L. Dunn on Nov. 24, 1970 and which describes a battery protection circuit which includes a power transistor between the battery and the load; U.S. Pat. No. 3,576,488 which issued to W. J. Zug et al on Apr. 27, 1971 and describes a battery discharge monitor for the terminal voltage of a battery for industrial trucks; U.S. Pat. No. 4,086,525 which issued on O. N. Ibsen et al on Apr. 25, 1981, and which teaches a circuit that senses the rate of discharge and determines a safe discharge voltage for a battery; and U.S. Pat. No. 4,280,097 which issued to R. L. Carey et al on July 21, 1981, and which teaches a system for monitoring the dc voltage of a source while electrically isolated from it.
These systems, though useful for particular applications, do not have all of the attributes desired in a current switch. These include a simple control circuitry, a low quiescent power dissipation, a low "on" resistance of the switch i.e. less than 0.25.OMEGA. from half to full load, a low value between "on-off" load voltages, i.e. &lt;2.5 V, a high speed short circuit protection of the switch, and an electrical isolation between load and control circuitry.